Have used Pens for 40+ years. Strangly, the most reliable seems to be the first one. It is a viewfinder camera with a scale focusing 4 element 28mm f3.5 lens stopping down to f22. These were fully manusl with no built in meter and are compact, front to back. This model has a 2 blade Copal leaf shutter with speeds of B-1/25-1/50-1/100-1/200 that is X synced with a PC connection and cold shoe. The lens will scale focus to a little less than 2 feet and with a 28mm lens at a diatance of over 5 feet DOF will cover minor errors in estimating distance.
The finder is a projected bright frame with close focus correction marks.
My personal experience is that many of the 2 blade shutters are still running fine, even after 50+ years. Just last year I obtained a early SS Pen made in late April early May 1960, just 7 months after production began at Sanko Shoji, a subcontractor set up to produce the first Pen until Olympus took over production in June of 1960.
I own or have owned several other all manual viewfinder Pens including the Pen S 2.8 with a 30mm, 4 element lens in a Copal 5 blade shutter with speeds from 1/8 to 1/250 plus B , X synced. I also have a Pen S 3.5 with the 28mm f3.5 lens in the same shutter. In my experience the Copal 5 blade shutters with extended speed selections are far more prone to be found gummed up and jammed. And, they do not flood clean well but have to be taken down, the blades cleaned, put back together, the speed controller pivots very carefully re-lubed and then final assembly back into the camera and the lens reset to infinity.
I also have a Pen F, about a 1965 production sample. this is the first model, without built in meter and 2 stroke film advance and shutter charge. There were two later models for non scientific use, the FT (built in meter) and FV (no built in meter) These later Pen F had single stroke film advance, and centered tripod bush (the first model had the tripod bush way over on the right side of the camera.)
Pen F cameras are all old and complex, be sure to check them carefully. On the single stroke film advance models make sure that the shutter will always be fully cocked with ONE STROKE, some worn Pen's take a full regular stroke than a small extra push on the advance lever before the shutter will fire. In my opinion whether the meter works or not on the FT is a matter of complete indifference. It was a wonky metering syatem anyway. If I had an otherwise good FT model I'd remove the meter with it's half silvered mirror entirely and replace it with a full front surfaced mirror and gain 1/2 stop brightness in the viewfinder.
A lot to absorb I know. For a pocket camera the first model is still fairly well available used. It was made from Oct/1959 to about mid 1965. I personally would avoid all Pen EE models. The ones with the 28mm f3.5 lens are fixed focus and the EES model with the 30mm f2.8 lens has scale focusing. You can manually select the aperture but on that setting the shutter speed is fixed at 1/40, just not a versitile as the full manual control models.
Don't get me started on the viewfinder type Pen D models.